1. Field of the Invention
The present invention relates to a piston for a reciprocating internal combustion engine, made of a nitride-forming base alloy and having a piston upper part, which is implemented having multiple peripheral piston ring grooves and a head land, and at least whose uppermost ring groove in proximity to the combustion chamber has hardened groove flanks protected against wear.
2. Description of the Related Art
The piston upper part, which represents the uppermost element diametrically opposite the combustion chamber and which passes through the entire combustion sequence, forms the most endangered part of a piston according to the species.
The seal against the combustion pressure acting from above on the piston upper part plays a special role here. This is produced by so-called piston rings, which are seated in the lateral part of the piston in piston ring grooves and which seal the combustion chamber toward the bottom in relation to the crankcase. A large problem in this area is the service life of the piston ring grooves. This determines the maintenance intervals and thus the operating costs and operational reliability of the entire piston very decisively.
These piston upper parts and in particular their piston ring grooves are known to be hardened to lengthen the operational lifetime, either inductively and/or by chroming. Inductive hardening results in moderate wear behavior. For example, DE 198 33 825 C1 has already disclosed a piston as described above, in particular for a large engine, in which the piston ring grooves have flanks protected against wear.
The piston upper part comprises tempered and alloyed steel here, which is hardened in the area of the upper and lower flanks of the piston ring grooves in such a way that continuous hardened zones result over the supporting area of the flank width. The hardening procedure may expediently be performed inductively, which permits the required temperature control to produce the desired microstructure conversion in a simple way.
However, this measure is no longer adequate beyond a specific level of the ignition pressure. The flank load increases disproportionately with increasing ignition pressure. The currently desired low lubrication rates are also unfavorable in this context. Therefore, rapid wear of the hardened flanks occurs. This is true in particular for the first piston ring groove lying closest to the combustion chamber, where experience has shown the load is greatest.
On the other hand, it is also already known that the wear resistance is significantly increased in nitrated or nitro-carburized steels.
In general, plasma nitration or plasma nitro-carburization is understood as hardening of the surface layers of steels, nitrogen and/or carbon atoms diffusing in and reacting in a thin surface layer with iron to form nitrides or carbon nitrides, the bonding layer. In the adjoining diffusion layer, the nitrogen is first partially precipitated as a nitride upon cooling and then causes the hardness increase. The hardness itself is a function of the type of the nitrides. Nitration times and layers differ depending on how the nitrogen is caused to react with the steel.
In other words, there is diffusion saturation of the boundary layer of a material with nitrogen to increase hardness, wear resistance, fatigue strength, or corrosion resistance. The boundary layer comprises an external nitride and/or carbon nitride layer (bonding layer) and an adjoining layer made of mixed crystals enriched with nitrogen and precipitated nitrides (diffusion layer) after the nitration/nitro-carburization.
The nitration times may be shortened by ionization of the nitrogen by glow discharge, so-called plasma nitration (plasma nitration at 450° C. to 550° C.).
In nitro-carburization, in which the treatment agent also contains components discharging carbon in addition to nitrogen, nitro-carburization may be performed in powder, salt bath, gas, or plasma (plasma nitro-carburization at 500° C. to 590° C., preferably at approximately 520° C.).